Slurry phase polymerisation of olefins is very well known, wherein an olefin monomer and optionally olefin comonomer are polymerised in the presence of a catalyst in a diluent in which the solid polymer product is suspended and transported. Two or more reactors are typically used in such polymerisations when it is desired to produce a multimodal product, in which a polymer made in a first reactor is transferred to a second reactor, where a second polymer having different properties to the first polymer is made in the presence of the first. However it may also be desirable to connect two reactors making monomodal polymers in order to create a swing monomodal/multimodal plant or to increase the flexibility of two small reactors that individually may lack the scale to be economically viable.
Slurry phase polymerisations are typically carried out at temperatures in the range 50-125° C. and at pressures in the range 1-100 bara. The catalyst used can be any catalyst typically used for olefin polymerisation such as chromium oxide, Ziegler-Natta or metallocene-type catalysts. The product slurry, comprising polymer and diluent and in most cases also catalyst, olefin monomer and comonomer can be discharged from each reactor intermittently or continuously, optionally using concentrating devices such as hydrocyclones or settling legs to minimise the quantity of fluids withdrawn with the polymer.
There are various options for transferring the active polymer slurry from one reactor to another. However, particularly in the case where the polymers being made in each reactor are different, the transfer is often difficult because it is desirable to control the polymerisation mixture in each reactor independently and therefore to remove from the stream exiting the first reactor any component that is undesirable in the second reactor. Several methods have been disclosed in the literature to operate such a transfer. For example, US 2001/0018499 describes a process in which most of the hydrogen present in the first reactor is removed by relieving the pressure of the suspension before sending it to the second reactor. U.S. Pat. No. 5,639,834 describes a process wherein a comonomer-rich suspension formed in the first reactor is drawn off from this reactor by means of settling legs, and a concentrated suspension sent to the second reactor in which the amount of comonomer has been reduced. U.S. Pat. No. 4,692,501 describes a process in which the suspension formed in the first reactor is washed by a liquid counter-current in an exchange zone before being sent to the second reactor. US 2001/0018500 describes a continuous polymerisation process wherein a suspension of polyethylene particles, withdrawn from the first reactor, is sent to a hydrocyclone separator where it is separated into a concentrated suspension that is sent to the second reactor, and a stream comprising the diluent that is partially recycled to the first polymerisation zone. There is no disclosure of the exact location of the hydrocyclone separator in the line between the first and second reactors.
However an additional problem with such transfers relates to fouling of or sedimentation in the transfer line between the two reactors under the wide range of flow conditions for which the line needs to be designed. The risk of such problems occurring is generally dependent on the velocity of the polymer-containing slurry and the associated pressure drop through the transfer line between the reactors, which depends on the line dimensions and configuration as well as the type of slurry being transferred. In large plants, where it may be desired to connect two reactors which are separated by a significant distance, the transfer line may have to be relatively long such that the effect of variations in pressure drop under differing transfer flows is increased. As a result, careful design of the transfer line between the reactors is required in order to maintain reliable polymer flow across the full range of desired operating conditions.
One solution to the above problem is to increase the velocity of the stream by using a smaller diameter line. But this can lead to an unacceptably high pressure drop along the line. We have found that the problems with sedimentation and fouling in the transfer line between a polymerisation reactor and a downstream vessel, particularly a second reactor, can be reduced by an arrangement in which the length of the transfer line through which the flow-rate of the slurry is the same as that entering the downstream vessel is minimised.